The invention relates to an inking unit for a printing machine and, more particularly, such an inking unit having a metering system for printing ink, and equipment for conveying printing ink on a conveyor path between the metering system and a plate cylinder of a printing machine. An inking unit of this general type is disclosed, for example, in the published German Patent Document DE 3 541 458 A1.
An important problem in the development and the operation of printing machines for offset printing is correct metering of dampening solution. Conventional printing machines use so-called inking units and dampening units to supply ink and dampening solution to a printing plate mounted on a plate cylinder. During a printing operation, unused dampening solution can penetrate into the ink travel path in the inking unit and dilute the printing ink emulsion. Particularly in the case of short inking units, because of the small number of splitting points between rollers and the small roller surface, the dampening solution cannot adequately emerge from the emulsion again. Especially in the case of printing from lightweight plates with little ink removal, but accordingly with a large quantity of dampening solution on the printing plate, the proportion of dampening solution in the emulsion can therefore increase over the course of the operation, which leads to printing problems and, under certain circumstances, to breakdown of the offset process. In order to counter this problem, the published German Patent Document DE 3 541 458 A1 includes a proposal to use a blower which directs an air flow onto a roller in the inking unit, in order to vaporize excessive dampening solution thereat. This leads to considerable air movements within the dampening unit and the printing machine, respectively, which can also have an effect upon the amount of dampening solution on other surfaces than that of the roller to which the flow is directed. These effects can be both additional evaporation at these other surfaces and, at another location, condensation of dampening solution evaporated at the roller to which the flow is directed. The type and extent of these effects can depend upon the changing climatic conditions in a room wherein the printing machine is set up. It is therefore difficult for an operator to control the output of the blower in a targeted manner and without previous testing so that a desired degree of drying of the emulsion is achieved.
Particularly if a heated airflow is used in order to reinforce the drying effect, considerable effects upon regions of the printing machine to which the flow is not directed cannot be avoided. In addition, the energy use in the case of such a procedure is considerable. Last but not least, the draft in the environment of the machine and the noise which is produced by such a blower is disturbing for personnel working at the printing machine.
It is accordingly an object of the invention, therefore, to provide an inking unit of the type noted in the introduction hereto wherein exactly reproducible control of the degree of drying of a roller is possible with a low expenditure of power and, in addition, any influence of the drying operation upon the climatic conditions on other surfaces of the inking unit, which carry dampening solution, or of a printing machine wherein the inking unit is used, is ruled out.
With the foregoing and other objects in view, there is provided, in accordance with the invention, an inking unit for a printing machine, having a metering system for printing ink, and equipment for conveying ink on a conveyor path between the metering system and a plate cylinder of a printing machine, comprising a vacuum chamber for evaporating a dampening-solution portion from the printing ink.
In accordance with another feature of the invention, the conveyor path passes through the vacuum chamber.
In accordance with a further feature of the invention, the vacuum chamber is bounded on a longitudinal side thereof by at least one roller of the ink-conveying equipment.
In accordance with an added feature of the invention, the vacuum chamber is also bounded by a hood-like cover extending along the roller and having longitudinal edges facing towards the roller.
In accordance with an additional feature of the invention, the longitudinal edges are sealed off by contact-free seals.
In accordance with an alternative feature of the invention, the longitudinal edges are sealed off by closing doctor blades.
In accordance with yet another feature of the invention, the closing doctor blades are engaged with the roller so as to be pressed against the roller by air pressure prevailing in the surroundings of the pressure chamber.
In accordance with yet a further feature of the invention, the roller is a screen roller, and the metering system comprises an inking chamber arranged on the screen roller.
In accordance with yet an added feature of the invention, the vacuum chamber directly adjoins the inking chamber.
In accordance with yet an additional feature of the invention, the vacuum chamber and the inking chamber are separated from one another by a common doctor blade.
In accordance with still another feature of the invention, the ink-conveying equipment includes at least another roller forming with the one roller a chain of mutually contacting rollers, at least the longitudinal side of the vacuum chamber being formed by the train of mutually contacting rollers.
In accordance with still a further feature of the invention, the vacuum chamber has end faces closed by respective plates.
In accordance with still an added feature of the invention, a liquid or plastic sealing material is held between the rollers and the plates by at least one of electrostatic and magnetic interaction.
In accordance with still an additional feature of the invention, the metering system includes a storage container, and the vacuum chamber is connected directly to the storage container.
In accordance with a concomitant feature of the invention, the vacuum chamber is disposed in a line for feeding ink back into a storage container of the metering system or for feeding ink from the storage container to an inking chamber on a roller.
The object of the invention is thus achieved with the aid of a vacuum chamber for evaporating a portion of the dampening solution from the delivered ink. The boiling point of the dampening solution, which is reduced in the vacuum chamber, promotes effective evaporation; dampening-solution vapor can be extracted via a pump used to generate the vacuum.
Although the published German Patent Document 4 225 451 A1 has already disclosed an inking unit for a printing machine having a suction pump connected to the metering system, this is not used to dry ink during running operation but to extract ink from the metering system when the machine is at a standstill, in the event of a color change.
The total quantity of gas to be conveyed is small in comparison with the air throughput of the heretofore known device. Influencing the behavior of the printing machine through vapor extracted from the vacuum chamber is therefore not very probable; this vapor is preferably discharged at a point where it cannot penetrate into the printing machine again, for example outside the room where the printing machine is set up.
The vacuum chamber is preferably arranged in such a way that the conveyor path passes through it. It therefore has the opportunity to influence the level of dampening solution in the ink directly before reaching the plate cylinder, i.e., immediately before the time which is critical for the quality of the print, or directly after contact with the impression cylinder, i.e., at a time whereat the dilution of the ink by dampening solution is relatively high.
According to a first possible construction, the vacuum chamber is bounded on a longitudinal side thereof by at least one roller belonging to the equipment for conveying ink, in other words the roller forms one wall of the vacuum chamber. Another wall of the vacuum chamber can be bounded by a hood-like cover, which extends along the roller and has longitudinal edges facing towards the roller.
In order to maintain a vacuum effectively in the chamber, the longitudinal edges are sealed off in relation to the roller, the type of seal that is selected being based upon the surface of the roller. In principle, the vacuum chamber can be arranged on any desired roller in the inking unit. If the roller has a soft surface which has little resistance to abrasion, use is preferably made of contact-free seals, in particular labyrinth seals, on the longitudinal edges. The same is true if the chamber is fitted to an ink applicator roller having the diameter of the plate cylinder. This is because, during printing, the relief of the subject is automatically formed on such an ink applicator roller, and this must not be destroyed by contact with seals of the vacuum chamber if it is wished to reliably avoid ghosting during printing.
If the surface of the roller adjoining the vacuum chamber is hard and/or a blurring of ink emulsion on the roller by seals of the vacuum chamber is not disruptive or is even possibly desired, in order to distribute the ink as uniformly as possible on the roller, use will expediently be made of contacting seals, for example in the form of a closing doctor blade. This construction is particularly expedient if the roller is a screen roller.
In this case, the closing doctor blades are preferably set against the roller in such a way that they are pressed against the roller by the air pressure prevailing in the surroundings of the chamber.
In this regard, the vacuum chamber can be provided upline or downline, as viewed in the direction of rotation of the screen roller, of an inking chamber likewise arranged on the latter and forming part of the metering system. A particularly space-saving possibility is to arrange the vacuum chamber directly adjacent to the inking chamber. In this case, a common doctor blade, which separates the vacuum chamber and the inking chamber from one another, can be used, on the one hand, to seal off the vacuum chamber with respect to the inking chamber, which is under a slight positive pressure during operation, and, on the other hand, for metering ink discharged from the inking chamber.
According to a second construction, the longitudinal sides of the vacuum chamber can be formed by a train of mutually contacting rollers. Because these rollers rotate together in contact with one another, they pick up ink emulsion from the conveyor path, the ink being distributed over the surface of all the rollers. In this way, virtually the entire inner surface of the chamber is used for vacuum-assisted drying.
The end faces of the chamber are closed off by plates. Gaps between the rollers and the plates are preferably sealed off by a liquid or plastic sealing material, which is kept in place by electrostatic or magnetic interaction.
In further possible constructions, provision is made for the vacuum chamber to be connected directly to the storage container for the printing ink, so that the ink in the container is continuously exposed to the vacuum, or for the vacuum chamber to be arranged in a line for feeding ink back into the storage chamber or for feeding ink to an inking chamber on a roller.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an inking unit for a printing machine, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, wherein: